1. Field of the Invention
The present invention relates to an organic light emitting element which has an anode, a cathode and a film comprising an organic compound in which light emission is obtained by applying an electric field (hereinafter referred to as organic compound film), and to a display device using the organic light emitting element. The present invention particularly relates to a display device including organic light emitting elements for emitting light of respective colors of red, green and blue as pixels, in which emission efficiency of the element for emitting red color light is high and also, the element life is long. Note that the display device in this specification indicates an image display device using an organic light emitting element as a light emitting element. Further, a module in which an organic light emitting element is attached to a connector, for example, an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape or a TCP (tape carrier package), a module in which a printed wiring board is provided at an end of the TAB tape or TCP, and a module in which an organic light emitting element is directly mounted with an IC (integrated circuit) by a COG (chip on glass) method, all are included in the display devices.
2. Description of the Related Art
An organic light emitting element is an element that emits light by applying an electric field. The light emission mechanism is described as follows. A voltage is applied to electrodes sandwiching an organic compound film, whereby an electron injected from a cathode and a hole injected from an anode recombine in the organic compound film to form a molecule in an excitation state (molecular exciton). Then, the molecular exciton releases energy in returning to a base state, to emit light.
In such an organic light emitting element, in general, the organic compound film is formed as a thin film with a thickness of less than 1 μm. Further, the organic light emitting element is a self light emitting element in which the organic compound film itself emits light, and thus does not need a backlight that is used in a conventional liquid crystal display. Therefore, it is a great advantage that the organic light emitting element can be extremely made thin and lightweight.
Further, for example, in the organic compound film with a thickness of approximately 100 to 200 nm, the time from carrier injection to carrier recombination is approximately several tens of nanoseconds with taking into consideration the carrier mobility of the organic compound film. Light emission is reached within microsecond even if the process from carrier recombination through light emission is considered. Therefore, it is one of strong points that a response speed is very high.
Furthermore, the organic light emitting element is a carrier injection type light emitting element. Thus, driving with a direct voltage is possible, and noise is hard to be occurred. As regards a driving voltage, there is the following report; first, the organic compound film is formed to be uniform and very thin with a thickness of approximately 100 nm; further, an electrode material which makes small a carrier injection barrier to the organic compound film, is selected; in addition, a hetero structure (here, two-layer structure) is introduced; thus a sufficient bright ness of 100 cd/m2 is realized at 5.5 V. (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,” Applied Physics Letters, vol. 51, No. 12, 913–915 (1987)).
Besides the above-described element characteristics such as thinness and lightness, high-speed respondence and direct low voltage drive, it is one of great advantages that the organic light emitting element has a large variety of emission colors. A factor for this advantage is the variety of the organic compound itself. That is, the flexibility that materials for various emission colors can be developed by molecule design (for example, introduction of a substituent) and the like leads to richness in colors.
The most applied field of the organic light emitting element which utilizes the richness in colors can be said to be a full-color flat panel display. The reason for this is that, full color can be easily attained by patterning the organic materials since there are a large number of organic materials capable of emitting the three primary colors of light of red, green and blue. The element characteristics such as thinness and lightness, high-speed respondence and direct low voltage drive as described above can be regarded as the characteristics suitable for the flat panel display.
By the way, white color can be obtained by emitting light of all the respective colors of red, green and blue. The balance of the three primary colors of light needs to be considered in emitting white color light. Thus, minimum required efficiency (here, power efficiency, the unit is 1 m/W) with respect to each color is shown (Reference 2: Yoshiharu Sato, “Applied Physics Society Organic Molecules—Bio-electronics Section,” Vol. 11, No. 1, P. 88 (2000)).
According to Reference 2, it is seen that there are a large number of reports in which a required value is exceeded as to green color and blue color while a value for red color falls far short of a required value. Therefore, the improvement in emission efficiency of red color is an essential element for developping of the full-color flat panel display. Then, the improvement in the emission efficiency enables reduction in power consumption.
It is given that a fluorescent material are used not only for a light emitting material for red color but also for a general organic light emitting element as one of factors in low emission efficiency. In the organic light emitting element, light emission is occurred when a molecular exciton returns to a ground state. The light emission from a singlet excitation state (S*) (fluorescence) and the light emission from a triplet excitation state (T*) (phosphorescence) are possible as the light emission. Only the light emission from S* (fluorescence) makes a contribution in the case where the fluorescent material is used.
However, a statistical generation ratio of S* to T* in the organic light emitting element is considered to be S*:T*=1:3 (Reference 3: Tetsuo Tsutsui, “Applied Physics Society Organic Molecules—Bio-electronics Section—Text of the Third Lecture Meeting,” P. 31 (1993)). Therefore, the theoretical limit of internal quantum efficiency (ratio of generated photons to injected carriers) in the organic light emitting element using the fluorescent material is established as 25% on the basis of S*:T*=1:3. In other words, in case of the organic light emitting element using the fluorescent material, at least 75% of injected carriers are wasted.
On the contrary, it is considered that the emission efficiency is improved (simply, three to four times) if the light emission from T*, that is, phosphorescence can be utilized. However, in a general organic compound, the light emission from T* (phosphorescence) is not observed at a room temperature, and only the light emission from S* (fluorescence) is generally observed. The reason for this is that, since the base state of the organic compound is generally a singlet ground state (So), T*→So transition is forbidden transition while S*→So transition is allowed transition.
However, the presentations on an organic light emitting element capable of converting energy released in returning to a ground state from T* (hereinafter referred to as “triplet excitation energy”) into light emission have been given one after another in recent years, and the highness of the emission efficiency has attracted attention. (Reference 4: D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “Improved energy transfer in electrophosphorescent devices,” Applied Physics Letters, vol. 74, No. 3, 442–444 (1999)), (Reference 5: Tetsuo Tsutsui, Moon-Jae Yang, Masayuki Yahiro, Kenji Nakamura, Teruichi Watanabe, Taishi Tsuji, Yoshinori Fukuda, Takeo Wakimoto and Satoshi Miyaguchi, “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center,” Japanese Journal of Applied Physics, Vol. 38, L1502–1504 (1999)).
A metal complex with platinum as central metal (hereinafter referred to as “platinum complex”) and a metal complex with iridium as central metal (hereinafter referred to as “iridium complex”) are used as light emitting materials in Reference 4 and Reference 5, respectively. It can be said that these metal complexes have such a feature that a third transition series element is introduced as the central metals. Both of the complexes are materials capable of converting triplet excitation into light emission at a room temperature (hereinafter referred to as “triplet light emitting material”).
As shown in Reference 4 and Reference 5, an organic light emitting element capable of converting triplet excitation energy into light emission can attain a higher internal quantum efficiency in comparison with prior art. Then, as the internal quantum efficiency becomes higher, the emission efficiency (1 m/W) is improved. Therefore, if the light emitting element for red color is manufactured by using the organic light emitting element capable of converting triplet excitation energy into light emission (hereinafter referred to as “triplet light emitting element”), the emission efficiency of the red color light emitting element can be improved.
From the above, an organic light emitting element that presents the light emission from a singlet excitation state (hereinafter referred to as “singlet light emitting element”) is used for green color and blue color while the triplet light emitting element is applied for red color, whereby the full-color flat panel display with sufficiently high brightness and low power consumption, in which the balance of the three primary colors of light is considered, is expected to be manufactured.
However, according to the report of Reference 5, the half-life of the brightness in constant current drive is approximately 170 hours when the initial brightness is set to 500 cd/m2, and thus, the triplet light emitting element has a problem on an element life. On the other hand, in case of the singlet light emitting element, the half-life of the brightness at constant current drive is several thousands of hours to ten thousands hours when the initial brightness is set at 500 cd/m2. Thus, it can be said that the singlet light emitting element reaches the practical stage in terms of an element life.
Therefore, in prior art, when the singlet light emitting element is used for green color and blue color while the triplet light emitting element is applied for red color to thereby manufacture the full-color flat panel display, a change of brightness in time largely differs between a pixel for green color or blue color and a pixel for red color.
Namely, this indicates that the balance of the three primary colors of light is greatly lost with the lapse of time (after several hundred of hours), and along with this, the power consumption in of light emission of red color increases. Therefore, it can be said that an extremely important technical object is to lengthen the life of the triplet light emitting element, particularly, the life of the triplet light emitting element for red color.